1. Description of the Related Art
The present invention relates to mobile communication via multiple wireless communication networks.
2. Description of the Related Art
In recent years, wireless networking systems have been developed that are capable of carrying local area Internet Protocol (IP) services, such as voice over IP (VoIP). For example, the 802.11b standard promulgated by the IEEE is a common standard that defines many aspects of networks that provide in-building wireless IP-based coverage. A single 802.11b access point provides a coverage area of about 100 meters in diameter. By networking these access points together in a grid, seamless coverage can be provided over a localized area to create a wireless local area network (WLAN).
Several companies have also developed wireless handsets that can be used to carry wireless voice traffic over such systems. For example, some handsets provide VoIP communications over 802.11b LAN installations using the ITU standard H.323. These handsets convert analog voice into compressed digital packets that are sent via the TCP/IP protocol over standard data networks.
FIG. 1 shows a network incorporating wireless voice over IP capabilities. A VoIP wireless phone 100 communicates encoded IP packets to one of the WLAN access points 102A-102N. The WLAN access points 102A-102N provide the physical footprint of the WLAN and pass the IP packets to and from a private branch exchange (PBX) telephone switch 104. If the PBX switch 104 is an IP-based device, it will directly accept the IP packets. If the PBX switch 104 is a legacy machine, a VoIP gateway (not shown) can be used to interface the access points 102A-102N to the PBX switch 104. The PBX switch 104 provides call control and routing functions. The PBX switch 104 can route calls either to a public switched telephone network (PSTN) 106 or over an IP backbone 108. Such systems also typically include a wired local area network 110 that provides service to wired desktop phones such as a desk phone 112. The wired local area network 110 may be IP-based, a legacy system or a combination of these.
If the PBX switch 104 is a part of an IP-based phone system, the PBX SWITCH 104 may use control signals, such as session initiation protocol (SIP), to provide call control processing. SIP defines the protocol mechanism necessary to provide call establishment, call forwarding, caller and called number delivery (often used to provide caller ID), remote unit capability negotiation, caller and called party authentication, caller and called device authentication, call transfer, conference calling and other calling features. However, other signaling mechanisms can also be used such as Skinny Station Protocol, which is Cisco's proprietary implementation of the H.323 IP telephony model. Using such a system, the wireless phone 100 can provide some of the same features available in the desktop phone 112 as the user wanders throughout the coverage area of the WLAN.
Unfortunately, once the user exits the coverage area of the WLAN, his wireless phone 100 is no longer capable of receiving WLAN calls. Rather than requiring a user to carry both a VoIP wireless phone for calls inside the WLAN coverage area and a cellular (e.g., GSM) phone for calls outside the WLAN coverage area, various manufacturers have proposed a dual-mode subscriber device capable of operating over both a WLAN and the cellular network. Such a device includes both a GSM chip set and a WLAN voice-over-IP chip set. Unfortunately, however, such a device also requires a dedicated call control processor (e.g., a SIP OR H.323 processor). This dedicated call control processor adds to the cost, size and complexity of the dual-mode subscriber device.